3GPP-LTE (3rd Generation Partnership Project Radio Access Network Long Term Evolution (hereinafter, referred to as “LTE”)) adopts OFDMA (Orthogonal Frequency Division Multiple Access) as a downlink communication scheme, and SC-FDMA (Single Carrier Frequency Division Multiple Access) as an uplink communication scheme (for example, see Non-Patent Literatures 1, 2 and 3).
With LTE, a radio communication base station apparatus (hereinafter abbreviated as “base station”) communicates with a radio communication terminal apparatus (hereinafter abbreviated as “terminal”) by allocating resource blocks (RBs) in a system band to terminals, per time unit referred to as “subframe.”
A base station also transmits, to terminals, downlink control information (L1/L2 control information) to notify the result of resource allocation for downlink data and uplink data. This downlink control information is transmitted to terminals using downlink control channels such as PDCCHs (Physical Downlink Control Channels). Here, each PDCCH occupies a resource comprised of one or more consecutive CCEs (Control Channel Elements). In LTE, the number of CCEs (CCE aggregation level) occupied by a PDCCH selects one of 1, 2, 4, and 8, according to the number of information bits of downlink control information or the condition of the propagation paths of terminals. Here, LTE supports a frequency band having a maximum width of 20 MHz as the system bandwidth.
In addition, when allocating a plurality of terminals to one subframe, a base station transmits a plurality of PDCCHs at the same time. At this time, the base station transmits a PDCCH including CRC bits masked (or scrambled) with a destination terminal ID to identify each PDCCH destination terminal. Then, a terminal performs blind decoding on a plurality of PDCCHs that may be directed to the terminal by demasking (or descrambling) CRC bits with the terminal ID of the terminal to detect the PDCCH directed to the terminal.
Downlink control information transmitted from a base station is referred to as “DCI,” and DCI includes such as resource information (resource allocation information) allocated from a base station to a terminal and MCS (Modulation and channel Coding Scheme). DCI includes a plurality of formats. That is, a plurality of formats mean an uplink format, a format for downlink MIMO (Multiple Input Multiple Output) transmission, a format for downlink discontinuous band allocation, or the like. A terminal needs to receive both downlink allocation control information (allocation control information on downlink) and uplink allocation control information (allocation control information on uplink). The downlink allocation control information includes a plurality of formats (downlink allocation control information formats), and the uplink allocation control information includes one format (an uplink allocation control information format).
For example, in the downlink control information (DCI), formats having a plurality of sizes are defined according to a transmitting antenna control method and resource allocation method or the like in a base station. Among this plurality of formats, the size of a downlink allocation control information format (hereinafter, referred to as “continuous band allocation downlink format”) performing continuous band allocation is the same as that of an uplink allocation control information format (hereinafter, simply referred to as “continuous band allocation uplink format”) performing continuous band allocation. These formats (DCI formats) include type information (for example, a flag of one bit) indicating a type of allocation control information (downlink allocation control information or uplink allocation control information). Therefore, by checking the type information included in the allocation control information, a terminal can specify whether the allocation control information is downlink allocation control information or uplink allocation control information, even when the size of continuous band allocation downlink format is the same as that of continuous band allocation uplink format.
The continuous band allocation downlink format is referred to as “DCI format 0 (hereinafter, referred to as “DCI 0”),” and the continuous band allocation uplink format is referred to as “DCI format 1A (hereinafter, referred to as “DCI 1A”).” As described above, DCI 0 and DCI 1A are the same size and can be distinguished according to the type information. Therefore, in the following explanation, DCI 0 and DCI 1A are represented together as “DCI 0/1A.”
Other than a continuous band allocation downlink format and a continuous band allocation uplink format, the DCI formats includes a downlink allocation control information format (“discontinuous band allocation downlink format”: DCI format 1: DCI 1) performing discontinuous band allocation, a downlink allocation control information format (“spatial multiplexing MEMO downlink format”: DCI formats 2 and 2A: DCIs 2 and 2A) allocating spatial multiplexing MIMO transmission and the like. Here, DCIs 1, 2, and 2A are the formats used depending on a downlink transmission mode (discontinuous band allocation or spatial multiplexing MIMO transmission) of a terminal. That is, DCIs 1, 2, and 2A are the formats set for each terminal. Meanwhile, DCI 0/1A is the format not depending on a transmission mode and used for a terminal of any transmission mode. That is, DCI 0/1A is the format commonly used for all terminals. When DCI 0/1A is used, one antenna transmission or transmit diversity is used as a default transmission mode.
Also, a method of limiting the CCEs subject to blind decoding on a per terminal basis has been studied to reduce the number of blind decoding attempts in order to reduce a circuit scale of a terminal. In this method, a CCE region (hereinafter, referred to as “search space”) that may be subject to blind decoding by each terminal is limited. Here, a unit of a CCE region allocated to each terminal (that is, equivalent to a unit to perform blind decoding) is referred to as “downlink control information allocation region candidate (PDCCH allocation region candidate)” or “blind decoding region candidate.”
In LTE, a search space is randomly set for each terminal. The number of CCEs forming this search space is defined per a CCE aggregation level of PDCCH. For example, the numbers of CCEs forming a search space become 6, 12, 8, and 16, associated with the CCE aggregation levels of PDCCHs 1, 2, 4, and 8, respectively. In this case, the numbers of blind decoding region candidates become six candidates (6=6÷1), six candidates (6=12÷2), two candidates (2=8÷4), and two candidates (2=16÷8), associated with the CCE aggregation levels of PDCCHs 1, 2, 4, and 8, respectively. That is, the blind decoding region candidate is limited to sixteen candidates in total. By this means, each terminal needs to perform blind decoding only for a blind decoding region candidate group in a search space allocated to the terminal, and therefore can reduce the number of blind decoding attempts. Here, the search space of each terminal is set using a terminal ID of each terminal and a hash function that is a function to perform randomization. This terminal-specific CCE region is referred to as “UE specific search space (UE-SS).”
On the other hand, PDCCH also includes control information (for example, allocation information of a downlink broadcast signal and allocation information of a paging signal) (hereinafter, referred to as “control information for common channel”) that is reported to a plurality of terminals at the same time and is used for data allocation common to all terminals. In order to transmit control information for common channel, a CCE region (hereinafter, referred to as “common search space (C-SS)”) that is common to all terminals that should receive a downlink broadcast signal is used for PDCCH. Four blind decoding region candidates (4=16÷4) and two blind decoding region candidates (2=16÷8), with respect to CCE aggregation levels 4 and 8 respectively, that is, in total, only six blind decoding region candidates exist in a search space of C-SS.
In UE-SS, a terminal performs blind decoding for two sizes of DCI formats, such as a first kind of DCI format (DCI 0/1A) commonly used for all terminals and a second kind of DCI format (such as DCIs 1, 2, and 2A) depending on a transmission mode. For example, since in UE-SS the terminal performs blind decoding on sixteen blind decoding region candidates with respect to both the first kind of DCI format (DCI 0/1A) and the second kind of DCI format (such as DCIs 1, 2, and 2A) having different sizes, the terminal performs blind decoding thirty-two times in total.
Also, since in C-SS a terminal performs blind decoding on six blind decoding region candidates with respect to DCI 1A and DCI format 1C (hereinafter, referred to as “DCI 1C”) that is a format used for common channel assignment, the terminal performs blind decoding twelve times in total.
Here, DCI 1A used for common channel allocation and DCI 0/1A used for data allocation of an individual terminal are the same size but are distinguished each other by a terminal ID. Therefore, also in C-SS, a base station can transmit DCI 0/1A performing data allocation of an individual terminal, without increasing the number of blind decoding attempts in a terminal.
Also, the standardization of 3GPP LTE-advanced, (hereinafter referred to as “LTE-A”) which realizes much faster communication than LTE, has been started. LTE-A realizes a downlink transmission speed equal to or higher than the maximum 1 Gbps and an uplink transmission speed equal to or higher than the maximum 500 Mbps. For this reason, it is expected to adopt a base station and a terminal (hereinafter “LTE-A terminal”) that are capable of communication at a wideband frequency equal to or higher than 40 MHz. In addition, an LTE-Advanced system is required to accommodate not only LTE-A terminals but also terminals (hereinafter “LTE terminals”) supporting an LTE system.
In LTE-A, studies are underway to introduce MIMO transmission having up to eight antennas, in addition to MIMO transmission having up to four antennas supported by LTE. Also, studies are underway to introduce CoMP transmission designed to improve throughput of terminals of a cell edge. In CoMP transmission, joint processing and coordinated scheduling are reviewed. The joint processing is the technique such that a plurality of base stations transmit a signal in a coordinated manner to make a terminal receive a signal with stronger power. The coordinated scheduling is the technique that a plurality of base stations reduce interference exerted on a terminal in a coordinated manner. That is, in CoMP, it is possible to perform MIMO transmission assuming a plurality of base stations as a transmitting point. In CoMP, SU-MIMO spatially multiplexing for one terminal and MU-MIMO spatially multiplexing for a plurality of terminals are possible.
In LTE-A, as a DCI format commonly used in MIMO and CoMP transmissions, studies are underway to define two different formats such as a format capable of reporting spatial multiplexing transmission of Rank 2 or more and a format capable of reporting spatial multiplexing transmission of only Rank 1 (for example, see Non-Patent Literature 4).